1. Field of the Invention
The present invention relates to a transfer printer for printing information by feeding a paper sheet into between a stationary print head and a platen roller along with a transfer ribbon, and transferring ink from the transfer ribbon onto the paper sheet by means of the print head.
2. Description of the Related Art
In a known transfer printer such as a label printer, feeding of a paper sheet by means of a paper feed roller is performed in interlock with rotation of a platen roller and transfer of a transfer ribbon. In practice, while the paper sheet driven by the feed roller runs between the print head and the rotating platen roller, the transfer ribbon is passed between the print head and the platen roller by rotating of the platen roller. In other words, the paper sheet and the transfer ribbon pass together in overlay relationship between the print head and the platen roller.
Since the paper sheet and the transfer ribbon are fed together at once, if information is printed on part of a label posted on the paper sheet, or if the label has a non-printing section, that portion of the transfer ribbon which corresponds to the non-printing section of the label is wastefully transferred.
For preventing wasteful use of the transfer ribbon, the printer is generally provided with a transmission mechanism arranged between a platen drive mechanism for rotating the platen roller and a ribbon drive mechanism for supplying the transfer ribbon. The transmission mechanism transmits drive power from the platen drive mechanism to the ribbon drive mechanism and interrupts the power transmission. When a non-printing section of each label comes, the transmission of the drive power is interrupted to stop the running of the transfer ribbon, thereby avoiding wasteful use of the ribbon.
The transmission mechanism includes a transmission gear supported by a pivotable gear holder. In general, the transmission gear remains in mesh with an interlocking gear arranged for rotating in interlock with the action of the platen drive mechanism and also, is capable of engaging an input gear of the ribbon drive mechanism for feeding the transfer ribbon. The holder is urged by a spring for rendering the transmission gear in mesh with the input gear.
By rocking the gear holder against the spring force of the spring, the transmission gear can be disengaged from the input gear while being in mesh with the interlocking gear. The transmission gear meshing with the interacting gear. Thus, the transfer ribbon can be transferred in interlock with the feeding of the paper sheet by the drive power transmitted through the co-movement of the two gears and is stopped by disengaging the gears from each other.
However, the engagement between the transmission gear and the input gear is determined by the urging force of the spring and thus, deeply intersected with no backlash. Also, since the gear holder is rocked, the pivotal point of the gear holder is off the line extending between the centers of the transmission and input gears.
Accordingly, when the transmission gear is engaged with or disengaged from the input gear by the pivotal action of the gear holder while the platen drive mechanism is operated or the transmission gear is rotated, its teeth tend to bite the teeth of the input gear. As the result, loads exerted on the two gears become large and the operational life of the gears will be decreased.
In the conventional printer, for prevention of the foregoing drawback, the platen drive mechanism is stopped when the drive mechanism for feeding a transfer ribbon is connected and disconnected to the power train. More specifically, the transmission gear is engaged with or disengaged from the input gear while it stops rotating. This allows loads exerted on both the gears during the action of engagement and disengagement to be released by free rotating movements of the gears. However, the running of a paper sheet has to be interrupted when the feeding of the transfer ribbon starts or stops, so that the action of ribbon saving will take a considerable period of time.